1. Field of the Invention
The present invention relates to a device for detecting scan start of a laser beam printer, and more particularly to a scan start detecting device for a laser beam printer, capable of detecting a correct scan start position.
2. Description of the Prior Art
Referring to FIGS. 1, 2A and 2B, there is illustrated a light source controlling device of a conventional laser beam printer. As shown in the drawings, the scan start detecting device comprises a light source unit a including a laser diode 21 and a collimator 22, a scanning unit b including a motor 25 and a scan disc hologram 24, a scan start detecting unit c including a cylindrical lens 33 and a detector 34, and a light source control unit d for receiving a signal outputted from the detector 34 and controlling the laser diode 21 of light source unit a, based on the received signal.
In an operation of the scan start detecting device with the above-mentioned construction, laser beams emitted from the laser diode 21 of light source a are fed to the collimator 22 in which they are converted into approximately parallel beams. The beams are then fed to the scan disc hologram 24 of scanning unit b, via a pre-scan hologram 23.
The scan disc hologram 24 of scanning unit b rotates by the driving force of motor 25. The scan disc hologram 24 scans the laser beams while rotating.
The scanned laser beams are then fed to a photosensitive drum 31, via planar mirror 26 and 27, non-spherical mirror 28 and planar mirrors 29 and 30, as shown in FIG. 2A. The beams are also fed to a planar mirror 32. The planar mirror 32 varies an optical path along which the beams are fed, so that the beams pass through the cylindrical lens 33 of scan start detecting unit c and then enters the detector 34. An output signal from the detector 34 is received in the light source controlling circuit d which, in turn, controls the laser diode 21, so as to turn on or off it, based on the received signal.
In the conventional device, however, the optical path of laser beams may be varied due to a variation in diffraction angle caused by a variation in wavelength in the laser diode 21. Such a variation in optical path results in a positional variation in scan start point. As a result, it is impossible to detect a correct scan start point.
Furthermore, the cylindrical lens 33 employed in the scan start detecting unit c is expensive, since it is fabricated by grinding its opposite surfaces. As a result, there is a disadvantage of an increase in manufacture cost.